Field Evaluations of Lettuce Cultivars for Resistance to Fusarium Wilt: 2-Year Summary

Abstract

Fusarium wilt of lettuce was first recognized in Arizona in 2001. The pathogen, Fusarium oxysporum f. sp. lactucae, has been recovered from infected lettuce plants in 27 different fields during the last three years. This fungus is a soil-borne pathogen that can remain viable in soil for many years. Historically, control of Fusarium wilt on crops other than lettuce, such as tomatoes and melons, has been achieved by planting cultivars resistant to the fungal pathogen. Large scale field trials were conducted during the 2002-03 and 2003-04 production seasons to evaluate existing lettuce cultivars for their relative susceptibility to Fusarium wilt. Among virtually all tested lettuce cultivars, the severity of disease in the first planting (early September) was much higher than that observed in the second planting (mid October), which in turn was higher than that observed in the third planting (early December). Soil temperatures differed considerably among plantings. In 2002-03 (or 2003-04) the average daily soil temperature at the 4-inch depth ranged from 65 to 85°F (70 to 94°F), 55 to 74°F (47 to 78°F), and 48 to 64°F (47 to 74°F) for the first, second and third plantings, respectively. In all three plantings, differences in disease severity were detected among the different types of lettuce, with head lettuce cultivars as a group being most susceptible and romaine cultivars collectively demonstrating the highest level of tolerance. Disease tolerance for specific cultivars was dependent on disease pressure. This is reflected in the comparative disease severity recorded in 2003-04 for specific cultivars (such as Beacon, Buccaneer, Coyote, Desert Heat, Lighthouse, Monolith, Red Tide, Sharpshooter, Sniper and Two Star) planted at each of the three different planting dates. Disease development began as early as the seedling stage and continued up to plant maturity, demonstrating the benefit of evaluating lettuce resistance in the field compared to greenhouse studies where plants are usually not carried to maturity before final disease ratings are performed. Data from these cultivar evaluation studies suggest that proper selection of planting date and cultivar would allow successful production of lettuce in fields infested with Fusarium oxysporum f. sp. lactucae.